"The NYPL Map Rectifier is a tool for digitally aligning ("rectifying") historical maps from the NYPL's collections to match today's precise maps. Visitors can browse already rectified maps or assist the NYPL by aligning a map. Play the video above to tour the site and learn how to rectify a map yourself."

Everyone is welcome to participate in aligning the maps but even if you're not into it you can spend hours just browsing. You'll see maps that describe what New York looked like in the past. Very fun!